I bought unfinished wooden hearts, circles, and rectangles from Hobby Lobby and/or Michael's. I painted them, and put cute stickers on them.
Since the letter stickers are flat, I painted over them with a layer of mod podge. (the other stickers are puffy, so I left them as is). I glued a strong magnet to the back of each decorative piece using E 600 glue. These magnets make me pretty nervous since I have a baby who puts everything in her mouth, so I used super strong glue and will check the necklaces often to make sure the magnets are secure. I made a set of magnettis for O and for my niece.